Since Opera 55 navigation menus in websites do not function correctly.

For example, find a menu with on-hover dropdown navbar items. If the pages under a main menu item tab display on-hover, Opera ignores clicks on the dropdown items. This is evident in all sites with on-hover menu dropdowns.

Any page with an on-hover menu, which does not include for example this page. Give us a URL to test it out on. (Something "safe for work" of course.)

As far as OS - in the case of Linux you should include distro (Ubuntu, Debian, etc.), version and desktop (if the distro has more than one available - examples would be Gnome, Mate, LXDE, etc.) Come to think of it, a number of distros are now switching to Wayland instead of Xwindows, so that might be something to mention as well. (Even with Windows you need to specify version, as there are big differences between Windows 7 and Windows 10, but in Linux it is harder.)

It seems to be intermittent. I'm not sure what triggers the fault. A site's menu might work fine the first few pages then suddenly the secondary nav tabs become non clickable.

I work with a lot of websites where I have to test each one's UX so I get to see this fault frequently.

This might be a related issue. Some sites lose keyboard page scroll functionality midway through a session. Again, it is an intermittent bug. The up and down arrow keys will work great for a couple of pages within a site then suddenly cease to function in their up/down scroll capacity. Scroll bar functionality continues to work. Mouse wheel continues to work. Keyboard up/down arrow keys cease to function. Keyboard otherwise works fine.